(1) Field of the Invention
The present invention relates to a multi-layer drawn plastic vessel. More particularly, the present invention relates to a multi-layer drawn plastic vessel comprising a laminate having a propylene resin layer and an oxygen-barrier layer formed of an ethylene/vinyl alcohol copolymer, in which the adhesion strength between both the resin layers is prominently improved even though the propylene resin layer is molecularly oriented.
(2) Description of the Prior Art
A laminate formed by bonding a layer of an olefin resin excellent in the moisture resistance, such as polypropylene or polyethylene, to a layer of an ethylene/vinyl alcohol copolymer excellent in the oxygen barrier property through an adhesive layer composed of an acid- or acid anhydride-modified olefin resin is used in various fields as a vessel having an excellent content-preserving quality in the form of a bottle, cup or pouch.
In order to reduce the thickness of the vessel wall as much as possible, improve the rigidity, impact resistance and other mechanical properties of the vessel wall and also improve the transparency and gas barrier property, it is preferred that this multi-layer plastic material be molecularly oriented in at least one direction. Furthermore, plastic processing such as draw forming or biaxial draw-blow forming is advantageous over heat processing because the heat energy cost is low and the productivity is ordinarily high, and by this plastic processing, molecular orientation is naturally caused in the formed vessel wall.
A conventional laminate formed by using an acid- or acid anhydride-modified olefin resin as an adhesive layer has an excellent adhesion strength (peel strength) in the undrawn state, but with increase of the degree of the molecular orientation by drawing, the adhesion strength is gradually reduced and interlaminar peeling is observed in the as-prepared vessel or is readily caused by slight falling shocks. This tendency is especially prominent when polypropylene which is excellent over polyethylene in the rigidity and mechanical strength is used as the olefin resin for outer and inner layers and this polypropylene is oriented by drawing.
Various proposals have heretofore been made in connection with means for controlling reduction of the adhesion strength of a laminate as mentioned above at the draw-forming step. For example, Japanese Patent Publication No. 11585/81 proposes a method in which a blend of an acid- or acid anhydride-modified olefin polymer and a crystalline ethylene copolymer is used as an adhesive layer interposed between the polypropylene layer and the ethylene/vinyl alcohol copolymer layer and a crystalline ethylene copolymer having a melting point lower by 40.degree. to 110.degree. C. than the melting point of the polypropylene is used as the ethylene copolymer contained in the blend. According to this proposal, the interlaminar peeling resistance is improved, but if the polypropylene layer is highly molecularly oriented, it is impossible to prevent the interlaminar peeling completely.